The present invention relates to display devices, and more particularly, to displays utilizing polymer-based electroluminescent devices.
Polymer-based electroluminescent devices (PLEDs) have the potential for providing inexpensive alternatives to semiconductor-based LEDs. PLEDs may be fabricated by coating the appropriate surfaces with an organic polymer, and hence, do not require the use of high cost fabrication systems such as those utilized in the fabrication of semiconductor devices. A simple PLED may be constructed from an electroluminescent layer sandwiched between an electron injection electrode and a hole injection electrode. More complicated devices utilize electron and hole transport layers between the above mentioned electrodes and the electroluminescent layer.
In principle, PLEDs can be utilized to generate inexpensive displays that display a single fixed image of the type used in point of sale advertising. Such a display would be self-illuminating, and hence, would replace transparencies that are mounted on a light-box. If a conventional display is used, the image must be stored in a memory external to the display and the display operated in a mode in which the stored image is continuously scanned into the display. The pixels of the display must be individually addressable which further increases the cost of the display.
In spite of the lower construction costs inherent in PLEDs, multicolor displays based on such devices are still quite costly to fabricate. To construct a multicolor display, a patterned deposition of each of a plurality of electroluminescent compounds must be performed. In prior art fabrication systems, a series of masking operations is required to protect the areas that are not to receive a particular electroluminescent compound. The electroluminescent compound is then deposited using vapor deposition, dipping, or spin casting. The mask is then removed and the next mask constructed using conventional photoresist techniques. Each masking operation increases the fabrication cost and reduces the device yield. In addition, the equipment and expertise needed to operate that equipment is of the type found in semiconductor fabrication facilities. It would be advantageous to provide a system that could be operated by printers and the like.
Broadly, it is the object of the present invention to provide an improved display based on PLEDs and a method and apparatus for constructing the same.
It is a further object of the present invention to provide a display that may be more inexpensively fabricated than prior art displays.
It is a still further object of the present invention to provide an improved method for fabricating PLED displays that does not require expertise in semiconductor fabricating techniques.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The present invention is a method for fabricating an electroluminescent display and the substrate and apparatus used therein. The display is preferably constructed on a pre-constructed substrate that includes a flexible base layer having a conducting surface on one side thereof. The base layer is impermeable to oxygen and water. The substrate includes a plurality of wells defined by a barrier layer, each well having an electrode layer connected electrically with the conducting surface. A removable protective layer covering the wells protects the conductive layer from attack by oxygen and water prior to being utilized to make the display. The electrode layer preferably includes a material chosen from a group of metals having a work function lower than 3.5 eV. This metal can be, for example, an alkali, alkaline earth or rare earth metal. In some cases, instead of a low work function metal, a metal layer of Ag or Al can be used which is coated with a thin layer of suitable alkali or alkaline earth oxide or fluoride, such as CaO, Li2O, MgO, LiF, MgF2, CsF, or CaF2. In one embodiment, each of the wells also contains an electron transport layer in contact with the electrode layer. The electron transport layer includes a material that improves the efficiency of the injection of electrons from the electrode layer into the electroluminescent layer of the display. The display is fabricated by moving a first dispenser relative to the substrate so as to quantitatively deposit a first electroluminescent material on the substrate, different amounts of the first electroluminescent material being deposited at different locations on the substrate in response to signals defining an illumination pattern to be generated by the display. A conducting material is deposited on the first electroluminescent material. In color displays, a second electroluminescent material is also deposited, the second electroluminescent material emitting light at a different wavelength than the first electroluminescent material. In the preferred embodiment of the present invention, the conducting materials include a hole transport material that facilitates the injection of holes into the first electroluminescent material. A non-luminescent material, which is preferably an electrical insulator, can be deposited in wells at locations on the display that are not to emit light prior to depositing the conductive material.